As a landlord do we have to carry out fire drills in general needs blocks of flats? 

No, regardless of whether it's stay put or full evacuate.

Why not? Can you provide a bit more detail please. 

Thank you

It's quite clear in the Government Guidance:

"While fire drills and practice evacuations are used in many buildings to reinforce fire awareness training, it is neither practical nor necessary to carry them out in purpose built blocks of flats. Even in blocks with communal fire alarm systems, this is unrealistic.
In large sheltered housing schemes incorporating extensive communal amenities, such as hairdressers, cafeterias and shops, fire drills may be necessary. However, these will still only apply to people present in the common parts. Residents within their flats would not be expected to take part in fire drills"
 

They are unnecessarily intrusive and disruptive to people who are, at the end of the day, in their own private dwelling, not an institute and on the whole are either ignored or result in damage to fire alarm sounders to silence the racket.

Being private dwellings there is no responsibility on occupiers to log whether  they are in or out and so it's impossible to know when to end the drill and timings as you can't be sure if everyone is out or not, most flat blocks are unstaffed with no one to coordinate evacuation . Also most blocks are stay put so no one should leave anyway (not that there should be sounders in the first place.)

It affords no benefit and is a waste of time and money. I've only ever tried it once and after leaving the alarm sounding for half an hour there was still no one leaving even though there were many flats occupied. The only result was two sounders smashed off the wall and several more bunged up with toilet roll.

This is interesting as the Fire officers are demanding a Fire Drill on our residential block of flats which I also think is a waste of time as we have no idea whose in or out and to be honest no one will take any notice of it. Should I just notify all staff and all tenant that we are carrying out a fire drill if they want to participate and ensure the on site staff are doing what they should and any weakness highlighted . 

I've come across this too, sometimes I've pushed back citing guidance and been successful, other times I've gone ahead and after 15 minutes given in and reset the alarm as no one bothered to come out....

If it's a staffed site there may be some benefit for them to practice even if no one else takes part.